1. Field of the Invention
The present invention relates to an automobile TV antenna system which uses a high-frequency pickup, and more particularly, to an improved automobile TV antenna system which is capable of performing diversity reception of TV broadcast waves. The system uses a TV antenna in which reception is optimized by an appropriate switch-over between a plurality of pickups. The pickups detect the surface currents induced by the broadcast waves which are received by a vehicle body.
2. Description of the Prior Art
These days, TV sets are often installed in automobiles etc., and such TV sets are used not only for receiving TV broadcasts but also for indicating various data with respect to vehicles.
Such an automobile TV separates a receiving signal consisting of TV broadcast waves into a video signal and a sound signal, and when the vehicle speed is stationary or below a predetermined speed, namely, when the vehicle is in the parking mode, the TV outputs both the video signal and the sound signal, while when the vehicle is running at more than a predetermined speed the TV only outputs the sound signal.
In such a TV set, the reception performance varies in accordance with the running state of the vehicle, which produces the problem of intermittent deterioration of the picture quality.
That is, high-frequency waves in the VHFLO band (90 to 108 MHz), VHFHi band (170 to 220 MHz), or UHF band (440 to 770 MHz) (these frequencies are used in Japan), such as TV broadcast waves, have a strong tendency to suffer from rectilinear propagation, so that direct waves and other waves interfere with each other as a result of bouncing off buildings, hillsides or other obstacles and this phenomenon produces distortion. Furthermore, in the case of a vehicle which is parked, the disturbance created by the waves generated by passing vehicles can cause multipath noise such as momentary interruption of sound during TV broadcast wave reception. An automobile antenna system having only a single antenna is unable to reduce such multipath noise during of TV broadcast wave reception.
Furthermore, it is difficult to receive TV broadcast waves stably, even if the sound of the TV broadcast is received, by a conventional pole antenna, during movement of a car. This is true, because due to the directivity of the antenna, the reception performance antenna changes in accordance with the change in direction from which the broadcast waves are received as a result of the automobile changing direction.
To solve this problem, a diversity-reception type automobile TV antenna system has been known, in which at least two antennas are disposed on the vehicle body at predetermined intervals so that the receiving operation is automatically taken over by the antenna having superior reception by performance whereby the directivity of the antenna is improved and the amount of multipath noise is lowered.
However, a conventional antenna is generally a pole antenna which projects outwardly from the vehicle body, and although it is superior in performance in its own way, it always remains a nuisance from the viewpoint of vehicle body design.
Especially, when diversity reception is performed by an automobile TV antenna system, a plurality of antennas are required. As a result, the aesthetic appearance of the automobile is disadvantageously damaged, and the receiving performance is greatly deteriorated by electrical interference between the antennas.
Another type of improved TV antenna system has been proposed which detects the surface currents induced on a vehicle body by broadcast waves.
Although utilization of currents which flow on the vehicle may apparently be the most reliable and efficient means, experiments carried out heretofore have shown very unfavorable results.
The first reason why the surface currents on an ordinary vehicle body cannot be effectively utilized is that the value of the surface currents has proved to be lower than expected so that it has not been possible to obtain a detecting output from the surface currents on the roof panel of a vehicle body (the portion mainly used for detection) that is at a high enough level.
The second reason is that the surface currents often have noise mixed therein. This noise is mainly produced by the engine ignition system of the vehicle and its battery charging regulator system, and it leaks into the vehicle body during the operation of the engine. It is therefore impossible to realize reception of broadcast waves in a clear enough manner for this system to be put to practical use.
As described above, a TV antenna system which detects the currents induced on a vehicle body by broadcast waves is not always successful.
Thus, to date, the problems relating to the design of a pickup structure suitable for efficiently detecting the currents flowing on a vehicle body which are induced by broadcast waves and the question of how to arrange the necessary pickups in such a manner as to ensure a good S/N ratio for practical use have not been fully solved.
In particular, the high-frequency directional characteristics of a high-frequency pickup as an antenna is poor, and further multipath noise is likely to be produced in the reception of FM broadcast waves of high frequency such as those in the VHF band.